This proposal is based on our recent studies in the murine model of influenza that CD8+ effector T cells (Teff) exhibit a regulatory feature at the sit of infection through the production of an immuno- regulatory cytokine, IL-10. The long-term objective of this investigation is to delineate the cellular and molecular mechanisms by which IL-10-producing (IL-10+) CD8+ Teff are induced during influenza virus (IAV) infection. The major hypothesis of this proposal is that type I interferon (IFN-I) signaling in neutrophils controls the development of IL-10+ CD8+ Teff in vivo through the induction of IL-27. Two specific aims are proposed: Aim1: To examine the role of neutrophils in IL-27 production and the development of IL-10+ CD8+ Teff during IAV infection. We will first examine the capability of neutrophils to produce IL-27 at mRNA and protein level in response to IAV infection in vitro and in vivo. We will also investigate the effects of selective ablation of neutrophils on IL-27 production, the development of IL-10+ CD8+ Teff, as well as the development of pulmonary inflammation during IAV infection in vivo. Aim2: To explore the role of IFN-I in controlling IL-27 production by neutrophils and the development of IL-10+ CD8+ Teff during IAV infection. Using neutrophils isolated from WT or IFN-I receptor deficient mice (IFNAR1 KO), we will examine the impact of the absence of IFN-I signaling on the production of IL-27 by neutrophils in vitro. We will then infect IFNAR1 KO mice to examine the effects of the absence of IFN-I signaling on IL-27 production, the development of IL-10+ CD8+ Teff, as well as the development of pulmonary inflammation during IAV infection in vivo. Relevance statement Immune-mediated pulmonary inflammation and injury is a frequent outcome of IAV infection. Thus, defining the mechanisms that can potentially counter-regulate the overactive immune responses is of great interest for lung injury prevention during IAV infection. We recently showed that anti-viral CD8+ Teff infiltrating the infected lungs exhibit both effector function to clear virus and regulatory featre by producing IL-10 to control excessive inflammation. This raises the possibility that the selective induction of these specialized regulatory IL-10+ CD8+ Teff during IAV infection may act to clear the infectious virus and control the excessive pulmonary inflammation simultaneously. The outcome of the analyses outlined in this proposal should lay out foundation for new avenues to selectively promote the generation of IL-10+ CD8+ Teff and so serve as specific therapeutic modalities aim to promote viral clearance and dampen pulmonary inflammation at the same time.